Google Chrome version 50 was released to the browser's stable channel yesterday, and in addition to a handful of new features and security fixes, the update also ends support for a wide range of operating systems that have been supported since Chrome launched on those platforms. Windows XP, Windows Vista, OS X 10.6, OS X 10.7, and OS X 10.8 are no longer supported.
This shouldn't come as much of a surprise, since Google promised last November to end support for these older OS versions in April of 2016. Old versions of Chrome installed on these OSes won't stop working (for now), but they'll no longer receive updates and there's no guarantee that things like Google account sign-in and data syncing will continue to work.
If you're still using one of these operating systems, you have a couple of options. One is to upgrade to a newer OS, assuming your hardware can handle it. Security patches for Windows XP stopped in April of 2014, and patches for OS X 10.6 stopped a few months before that. Updates for OS X 10.7 and 10.8 ended roughly when versions 10.10 and 10.11 were released, respectively, since Apple's unofficial policy is to provide security fixes for the most recent OS X release and the two previous releases. Windows Vista is still getting bare-minimum security patches from Microsoft, but that ends in April of 2017.